The United States Department of Energy has set aggressive goals for increasing the use of wind power in the country. In order to achieve these goals, the Cost Of Energy (COE) must come down for wind power because it is not cost-competitive with other energy sources at this time. Currently, one of the most promising opportunities for significantly reduced COE is through the coordinated development of superior low-cost materials using reliable, high-volume component manufacturing techniques for components used in wind power applications. Rotors, usually consisting of two or three blades attached to a hub, represent the highest cost component of a wind turbine despite being less than 15% of its weight. Reducing blade weight has a dramatic weight-saving effect throughout the rest of the wind turbine. However, a careful balance must be achieved between reductions in blade weight and the higher costs typically associated with specialized lightweight materials such as carbon composites in order to realize reduced COE overall. Most large wind blades are currently made from glass fiber reinforced plastic (GFRP), with some sandwich core materials.
The value of lighter materials becomes a necessity when trying to scale to larger blades, and thus more efficient turbines. In scaling wind blade sizes from 40 m to 60 m in length, commercial blades at the upper end of the current size range are already nearing the limit of conventional designs from the standpoint of size, strength, and durability in operation over time. For large blades to avoid the near cubic weight increase with size, carbon or glass/carbon hybrid composites and manufacturing processes that yield better mean properties and/or reduced property scatter through improvements in fiber alignment, compaction and void reduction are required. This nonlinear increase of weight as a function of length/size was also discussed in references relevant in the art area.
While carbon fiber reinforced plastics (CFRP) have superior properties, such as about three times the stiffness with significantly better fatigue properties compared to GFRP, it also has a much higher cost. While commonly used E-glass costs less than USD$1/lb at the time of the application filing for letters patent of the present invention, standard modulus carbon fiber costs on the order of 6 to 20 $/lb, depending on type, tow-size and volume. As such, the present invention proposes a solution that provides a resultant product that is a hybridize of these two materials together to achieve an optimum design based on cost and performance. In general, the cost advantage or disadvantage of carbon fiber replacement will depend on the cost ratio of labor to materials. In order to take advantage of carbon properties compared with the prior art composites, new designs and manufacturing methods of the present invention provide for reduced labor time and therefore reduced costs, which now permit CFRP wind blades to be manufactured at a commercially competitive cost. From an industrial point of view, advantages of carbon fiber reinforced plastics/composites in blades according to the present invention include:                1. Thinner and more efficient profiles resulting in higher energy output,        2. Stiffer blade resulting in shorter nacelle,        3. More slender blades resulting in lower extreme loads on tower and nacelle, and        4. Lower blade mass resulting in easier to handle production and mounting.        
The present invention includes materials, preferably 3-D woven hybrid glass-carbon and matrix materials, used in spar caps for wind blades, the spar caps made therefrom and the wind blades made therewith. Advantages of using these materials in such embodiments are discussed in the following in detail, and includes in summary: more efficient design approach using integral, unitary, single-piece variable width, with decreasing width from root to tip of the blade, spar caps and/or 3-D woven skins with variable density balsa behind and forward of the spar caps; and improved manufacturing processes including resin infusion of same, thereby leading to reduced labor cost and better quality control, as well as improved products and use thereof.
When it comes to core materials, Baltek Corporation of Northvale, N.J., USA is the world leader in balsa core materials. They have spent over 25 years of research perfecting end grain balsa materials. Genetically selected seeds are plantation grown in ideal conditions producing balsa trees with much improved consistency. Balsa core materials are used in many wind turbine blade designs today, and Baltek is a big player in this market. Current designs use only one density of core along the entire length of the blade. An improved structural efficiency can be obtained in a wind blade by tailoring the sandwich core to only the required density, which will vary along the length of the blade. To enable cost efficient manufacturing of the blades, the core materials can be pre-cut, labeled and “kitted” at the Baltek factory.
One of the most promising recently developed textile processes is a new form of 3D weaving being commercialized under the trademark 3-D woven by 3TEX, Inc. of Cary, N.C. USA. Embodiments of the present invention preferably include unitary, integral 3-D woven materials and/or 3-D weaving technology, as well as distinguishing its differences from 2-D weaving and previous 3-D weaving techniques.
A fully automated 3-D weaving process with multiple, simultaneous filling insertions was developed at the North Carolina State University College of Textiles, located in Raleigh, N.C. This process does not involve the building up of layers in the fabric; instead, a unit of thick, true 3-D fabric is formed during each weaving cycle.
There are at least three revolutionary advances contained within this process, including the automated use of multiple weft insertion in a single weaving cycle, the automated method of producing net-shaped forms in various cross-sectional shapes, including “I”, “T” and “P” shapes, as well as core or pile structures, and the ability to include controlled amounts of Z direction fiber, for example up to ⅓ of the total fiber volume, in an integral and automated fashion. Due to multiple filling insertions per weaving cycle, architectures can be achieved that cannot be done with conventional weaving. In addition to these advances, 3-D woven materials do not have internal fiber crimp, or interlacing at yarn intersections within the body of the material or fabric, which enhances fatigue performance over previously attempted conventional carbon materials in wind turbine blade applications. A schematic of the 3-D orthogonal woven structure is illustrated in FIG. 1.
The ability to make thicker/heavier fabrics with a controlled and uniform fiber architecture results in some inherent advantages for 3-D woven fabrics, including thicker fabrics for providing fewer required layers and less labor, faster resin infusion due to higher permeability for faster composite processing times, and low or no fiber crimp for higher in-plane properties (e.g. tension and compression), sufficient amounts of Z direction fibers providing for higher transverse shear strength and total suppression of delamination. Many of these advantages resulting from Z-direction fiber have been studied at the laboratory scale for years, as evidenced by the over 170 references cited in a review article. These advantages are realized in commercially available materials produced by 3TEX, Inc. of Cary, N.C. USA, which manufactures carbon and glass 3-D woven materials for applications in the marine and other industries.
The present invention applies these advantages with intelligent hybridization of carbon and glass for novel applications in wind blade spar cap design and construction, and methods therefore, which have not been taught or suggested in the prior art. Thus, prior to the present invention, there has remained a need in the art for hybrid carbon-glass composite spar caps for wind blades for providing increased stiffness and performance, with improved processing and reasonably competitive commercial costs.